Optical transmitter, optical receiver, optical transmission system, and optical transmission method

ABSTRACT

An optical transmitter in accordance with the present intention consists of a light source and an interference unit. The interference unit consists of an optical power divider, an intensity modulator, a gain variation device, a phase shifter, an optical coupler, and a phase controller. In the optical transmitter, continuous-wave light emanating from the light source is bifurcated by the optical power divider. One of resultant continuous-wave rays has the intensity thereof modulated based on transmission data by the intensity modulator. The other continuous-wave light has the power thereof adjusted by the gain variation device, and then has the phase thereof shifted by the phase shifter. The optical coupler joins the light signals, whereby the phase of part of the modulated light signal is shifted. The gain variation device is realized with a combination of, for example, an optical amplifier and an optical attenuator. The phase shifter is realized for example, a phase modulator and an optical delay device. A phase controller controls the phase shifter and gain variation device. A light signal emitted from the interference unit is transmitted over a transmission line that is not shown.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical transmitter, an opticalreceiver, an optical transmission system, and an optical transmissionmethod. More particularly, this invention is concerned with an opticaltransmitter, an optical receiver, an optical transmission system, and anoptical transmission method for improving the resistivity of an opticaltransmission system to wavelength dispersion.

2. Description of the Related Art

In high-speed optical fiber communication systems, waveform distortionderived from waveform dispersion occurring along an optical fiber thatis a transmission line is a factor of restricting a transmission rate ora distance of transmission. A transmission method making a systemresistive to wavelength dispersion and a wavelength dispersioncompensation technology are therefore indispensable.

Conventionally proposed transmission methods making a system resistiveto wavelength dispersion include an optical duobinary method. For thedetails of the optical duobinary method, refer to “Characteristics ofOptical Duobinary Signals in Terabits Capacity, High-spectral EfficiencyWDM Systems” (Journal of Lightwave Technology, Vol. 16, No. 5, pp.788-797, May 1998).

According to the optical duobinary method, a binary signal is convertedinto a ternary signal or any other multilevel signal in order tocompress the spectrum of an electric signal. The spectrum of theelectric signal is compressed to agree with approximately a half of thespectrum of a non-return-to-zero (NRZ) signal to be transmitted at thesame bit rate as a bit rate at which a light signal proportional to theelectric signal is transmitted. Consequently, compared with the NRZsignal to be transmitted at the same bit rate, the light signal whosespectrum has been narrowed according to the optical duobinary method ispermitted to disperse by approximately twice a larger magnitude whiletraveling over an optical transmission line.

However, a precoder circuit employed according to the optical duobinarymethod delays an output of an exclusive OR circuit by a time duringwhich a data signal represents one bit, and feeds it back to one inputterminal thereof. As a transmission rate increases, higher precision isrequired to adjust a delay time. This poses a problem in that it becomesdifficult to realize the precoder circuit.

According to the optical duobinary method, an electric signal isconverted into a multilevel signal. This poses a problem in that theconfiguration of a currently commercially available transmitter/receiverfor transmitting or receiving the NRZ signal must be modified oroptimized.

SUMMARY OF THE INVENTION

An object of the present invention is to solve the foregoing problemsand to provide an optical transmitter, an optical receiver, an opticaltransmission system, and an optical transmission method making itpossible to obviate a precoder circuit. Moreover, with the opticaltransmitter, optical receiver, optical transmission system, and opticaltransmission method in accordance with the present invention, a codingmethod using a non-return-to-zero (NRZ) signal or a return-to-zero (RZ)signal as a carrier and making an optical transmission system highlyresistive to dispersion can be realized.

An optical transmitter according to the present invention comprises anoptical power divider, a light modulator, a phase shifter, and anoptical coupler. The optical power divider divides input light intofirst and second continuous wave (CW) lights. The light modulatormodulates the first CW light according to a data signal so as to outputmodulated light. The phase shifter shifts the phase of the second CWlight so as to output phase-shifted light. The optical coupler couplesthe modulated light and phase-shifted light.

An optical receiver according to the present invention comprises a firstphoto-detector, a clock extracting circuit, and a maximum leveldetection circuit. The first photo-detector converts a first lightsignal to be input into an electric signal. The clock extracting circuitextracts a predetermined frequency component of the electric signal andoutputs it as a clock signal. The maximum level detection circuitdetects the maximum level of the clock signal.

An optical transmission system according to the present inventioncomprises an optical transmitter, an optical transmission line, and anoptical receiver. The optical transmitter outputs a light signal. Thelight signal is transmitted over the optical transmission line. Theoptical receiver receives the light signal output over the transmissionline. The optical transmitter comprises the foregoing opticaltransmitter, and the optical receiver comprises the foregoing opticalreceiver. The optical transmission line includes a control signaltransmission line over which the maximum level is transmitted to theoptical transmitter.

An optical transmission method according to the present inventioncomprises four steps. At the first step, light emanating from a lightsource is divided into at least two division lights. At the second step,the phase of one of the division lights is shifted in order to producephase-shifted light. At the third step, the other division light ismodulated in order to produce modulated light. At the fourth step, thephase-shifted light and modulated light are coupled.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawingswherein:

FIG. 1 is a block diagram showing the configuration of an opticaltransmitter in accordance with the first embodiment of the presentinvention;

FIG. 2 is a flowchart describing a procedure of setting phase shiftemployed in the first embodiment of the present invention;

FIG. 3 is a graph expressing the advantage of the optical transmitter inaccordance with the first embodiment of the present invention;

FIG. 4 is a block diagram showing the configuration of an opticaltransmitter in accordance with the second embodiment of the presentinvention;

FIG. 5 is a block diagram showing the configuration of an opticaltransmitter in accordance with the third embodiment of;the presentinvention;

FIG. 6 is a block diagram showing the configuration of an opticaltransmitter in accordance with the fourth embodiment of the presentinvention;

FIG. 7 shows the configuration of a light modulator shown in FIG. 6;

FIG. 8 is a block diagram showing the configuration of an opticaltransmission system in accordance with the fifth embodiment of thepresent invention;

FIG. 9 is a flowchart describing a procedure of setting a phase shiftemployed in the fifth embodiment of the present invention; and

FIG. 10 is a block diagram showing the configuration of an opticaltransmission system in accordance with the sixth embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, embodiments of the present invention will be described withreference to the drawings. FIG. 1 is a block diagram showing theconfiguration of an optical transmitter in accordance with the firstembodiment of the present invention. Referring to FIG. 1, an opticaltransmitter 1 comprises a light source 11 and an interference unit 12.The interference unit 12 comprises an optical power divider 13, anintensity modulator 14, a gain variation device 15, a phase shifter 16,an optical coupler 17, and a phase controller 18.

In the optical transmitter 1, continuous-wave light emanating from thelight source 11 is bifurcated by the optical power divider 13. One oftwo resultant continuous-wave lights has its intensity modulated basedon transmission data by the intensity modulator 14. The othercontinuous-wave light has its power adjusted by the gain variationdevice 15, and has its phase shifted by the phase shifter 16. Theoptical coupler 17 couples the light signals. Thus, part of a modulatedlight signal has it phase shifted.

The gain variation device 15 may be realized with a combination of, forexample, an optical amplifier and an optical attenuator. The phaseshifter 16 may be realized with, for example, a phase modulator and anoptical delay unit. The phase controller 18 controls the phase shifter16 and gain variation device 15. A light signal emitted from theinterference unit 112 is transmitted over a transmission line that isnot shown.

According to the foregoing configuration, the gain variation device 15is installed in the input stage of the phase shifter 16. The presentinvention is not limited to this configuration. Alternatively, the gainvariation device 15 may be installed in the output stage of the phaseshifter 16 or in both the input and output stages thereof. Moreover, thegain variation device 15 may be installed in an arm including theintensity modulator 14 instead of an arm including the phase shifter 16,or may be installed in both the arms.

An advantage to be provided by the present embodiment will be describedby introducing concrete formulas. Assume that the spectrum of anelectric signal used as an intensity modulating signal is G(f). A lightsignal modulated based on the electric signal having the spectrum G(f)is propagated over an optical transmission line causing wavelengthdispersion of a magnitude D, and then photoelectrically converted. Thespectrum S(f) of an electric signal resulting from photoelectricconversion is expressed as follows:

S(f)=α×cos(πf ²λ² D/c)G(f)+β(f)

where λ denotes the wavelength of light output from the light source, cdenotes a light velocity, f denotes a frequency, α denotes acoefficient, and β denotes a correction term. β(f) is much smaller thanthe first term in the right side. As the magnitude D of wavelengthdispersion increases, S(f) becomes more dependent on cos(πf²λ²D/c)G(f),or in other words, the waveform of the electric signal deterioratesmarkedly.

According to the present embodiment, the phase of part of light outputfrom the light source is shifted by Δφ. In this cause, the spectrum S(f)of the electric signal resulting from photoelectric conversion isexpressed as follows:

S(f)=α×2cos(Δφ/2)

cos(πf ²λ² D/c−βφ/2)G(f)+γ(f)

where γ(f) is a correction term and much smaller than the first term inthe right side. As apparent from the formula, πf²λ² D/c−Δφ/2 containedin the first term in the right side is converged to 0 by properlyadjusting Δφ. Therefore, an increase in the magnitude D of wavelengthdispersion can be compensated with the phase shift Δφ. Consequently,deterioration in the waveform derived from waveform dispersion can besuppressed.

FIG. 2 is a flowchart describing how to set a shift to be provided bythe phase shifter 16 in the optical transmitter 1. FIG. 3 is a graphexpressing the advantage provided by the optical transmitter 1 inaccordance with the first embodiment of the present invention. Referringto FIG. 1 to FIG. 3, a description will be made of a procedure ofsetting the phase shift to be provided by the optical transmitter 1.

To begin with, the gain variation device 15 is adjusted so that thepower of a light signal output from the intensity modulator 14 will beagreed with the power of continuous-wave light output from the phaseshifter 16. Thereafter, the phase φ1 of a carrier that is a light signaloutput from the intensity modulator 14 is matched with the phase φ2 ofthe continuous-wave light output from the phase shifter 16 (step S1 andS2 in FIG. 2).

On the assumption that the magnitude D of wavelength dispersionoccurring over the transmission line is already known, the phase of aphase shifter output signal light is shifted by Δφ(=(1/3)×10⁻⁵×(λf)²D×π) using the phase shifter 16 (step S3 and S5 inFIG. 2).

In the above formula, λ[μm] denotes the wavelength of continuous-wavelight output from the light source 11. D[ps/nm] denotes the magnitude ofwavelength dispersion occurring over a transmission line, and f[GHz]denotes a frequency. According to the present invention, the frequency fis set to a half of a clock frequency. When Δφ exceeds 2π/3, it is setto 2π/3. When Δφ is equal to or smaller than −2π/3, it is set to −2π/3.When the magnitude D of wavelength dispersion occurring over atransmission line is unknown (step S3 in FIG. 2), D is measured (step S4in FIG. 2).

According to the present embodiment, a transmission rate of data carriedby signal light is set to 40 Gbps. The intensity modulation method isbased on the non-return-to-zero (NRZ) method. A single-mode fiber isused as the transmission line.

An advantage to be provided by the present embodiment will be describedin conjunction with FIG. 3. The axis of abscesses in FIG. 3 indicatesmagnitudes of dispersion occurring over the transmission line, and theaxis of ordinates indicates power losses depicted as eye openingdegradation in an eye pattern. In FIG. 3, a curve indicated with“present invention” is plotted with the results of calculation of themagnitude D of wavelength dispersion. For calculating the magnitude ofpositive dispersion, 0.92π is assigned to Δφ. For calculating themagnitude of negative dispersion, 0.08π is assigned to Δφ. For compareson, FIG. 3 also shows a curve plotted with the results of calculationof the magnitude D of wavelength dispersion to be exhibited by a lightsignal modulated according to the ordinary, NRZ method. As apparent fromFIG. 3, compared with the NRZ method, a coding method employed in thepresent embodiment makes an optical transmission system resistive todispersion of about twice a larger magnitude. The present embodiment maybe combined with any other coding method, for example, thereturn-to-zero (RZ) method. Even in this case, an optical transmissionsystem will be resistive to dispersion of about twice a largermagnitude.

FIG. 4 is a block diagram showing the configuration of an opticaltransmitter in accordance with the second embodiment of the presentinvention. Referring to FIG. 4, an optical transmitter 2 comprises alight source 21 and an interference unit 22. The interference unit 22comprises an optical power divider 23, an intensity modulator 24, aphase shifter 26, an optical coupler 27, and a phase controller 28. Theoptical transmitter in accordance with the present embodiment isdifferent from the optical transmitter 1 in accordance with the firstembodiment in terms of the interference unit 22 alone.

A dividing ratio at which the optical power divider 23 divides a lightsignal is determined so that the mean powers of lights output from theintensity modulator 24 and phase shifter 26 respectively and coupled bythe optical coupler 27 will be equal to each other. Assume that lightlosses occurring through two paths that extend from the optical powerdivider 231 to the optical coupler 27 are equal to each other, and thata coupling ratio at which the optical coupler 27 couples two rays is1:1. In this case, the dividing ratio at which the optical power divider23 divides a light signal is set to 1:2. When the requirements arechanged, the dividing ratio at which the optical power divider 23divides a light signal is modified accordingly.

The present embodiment is characterized in that the coupling ratio atwhich the optical coupler 27 couples two lights is 1:1. The gainvariation device 15 included in the optical transmitter 1 in accordancewith the first embodiment of the present invention is excluded from thepresent embodiment. This leads to the more compact optical transmitter.

FIG. 5 is a block diagram showing the configuration of an opticaltransmitter in accordance with the third embodiment of the presentinvention. Referring to FIG. 5, an optical transmitter 3 comprises alight source 31 and an interference unit 32. The interference unit 32comprises an optical power divider 33, an intensity modulator 34, aphase shifter 36, an optical coupler 37, and a phase controller 38. Theoptical transmitter in accordance with the present embodiment isdifferent from the optical transmitter 1 in accordance with the firstembodiment in terms of the interference unit 32 alone.

The present embodiment is characterized in that a dividing ratio atwhich the optical power divider 33 divides a light signal is 1:1.Moreover, the optical coupler 37 couples two lights at a coupling ratioof 1:2. Continuous-wave light output from the phase shifter 36 andsignal light output from the intensity modulator 34 are coupled at 1:2.The coupling ratio is not limited to 1:2. However, 1:2 is the mostpreferable. The gain variation device 15 included in the opticaltransmitter 1 in accordance with the first embodiment is excluded fromthe present embodiment. This leads to the more compact opticaltransmitter.

FIG. 6 is a block diagram showing the configuration of an opticaltransmitter in accordance with the fourth embodiment. Referring to FIG.6, an optical transmitter 4 comprises a light source 41 and aninterference unit 42. The interference unit 42 comprises a lightmodulator 50 and a phase controller 48.

The optical transmitter in accordance with the present embodiment isdifferent from the optical transmitter 1 in accordance with the firstembodiment in terms of the interference unit 42 alone. The interferenceunit 42 is constructed in the form of one module serving as the lightmodulator 50.

FIG. 7 shows the configuration of the light modulator 50 shown in FIG.6. Referring to FIG. 7, the light modulator 50 comprises a modulationdevice 51, a modulation electrode 52, a ground 53, a dc bias electrode54, an optical waveguide 55, and a termination circuit 56.

A device exerting a great electro-optic effect and made of LiNbO₃ orLiTaO₃ is preferable as the modulation device 51. LiNbO₃ is morepreferable. A dividing ratio and coupling ratio at which a light signalis divided over the optical waveguide and lights are coupled over it arethe same as those employed in the second or third embodiment.

According to the present embodiment, the interference unit 12 includedin the optical transmitter 1 of the first embodiment is constructed inthe form of a module serving as the light modulator 50. This leads tothe more compact optical transmitter. Besides, more stable actions areperformed owing to the interference unit 52.

FIG. 8 is a block diagram showing the configuration of an opticaltransmission system in accordance with the fifth embodiment of thepresent invention. Referring to FIG. 8, the optical transmission systemin accordance with the present embodiment comprises an opticaltransmitter 6 and an optical receiver 7.

The optical transmitter 6 comprises, similarly to the opticaltransmitter 1 in accordance with the first embodiment, a light source 61and an interference unit 62. The interference unit 62 comprises anoptical power divider 63, an intensity modulator 64, a gain variationdevice 65, a phase shifter 66, an optical coupler 67, and a phasecontroller 68. The optical receiver 7 comprises a photoelectricconversion unit 71, a reproduction and identification circuit 72, ahalf-clock extracting circuit 73, and a maximum level detection circuit74.

A light signal transmitted from the optical transmitter 6 and propagatedover a transmission line 69 is converted into an electric signal by thephotoelectric conversion unit 71 included in the optical receiver 7. Theelectric signal is divided into two signals. One of two resultantelectric signals is converted into a clock signal and an identified andreproduced output signal by the identification and reproduction circuit72. The other electric signal is input to the half-clock samplingcircuit 73, whereby a half-clock frequency signal Is extracted. Thehalf-clock extracting circuit 73 is realized with, for example, anarrow-band filter.

The half-clock frequency signal extracted by the half-clock samplingcircuit 73 is input to the maximum level detection circuit 74. A controlsignal is then produced. The control signal produced by the maximumlevel detection circuit 74 is transferred to the phase controller 68included in the optical transmitter 6. An alarm signal transmissionelectric cable normally contained in the transmission line 69 ispreferably used to transfer the control signal.

FIG. 9 is a flowchart describing a procedure of setting a shift to beprovided by the phase shifter 66 and a gain to be produced by the gainvariation device 65. Referring to FIG. 8 and FIG. 9, a description willbe made of actions to be performed in the optical transmission system inaccordance with the present embodiment. Actions to be performed when themagnitude D of wavelength dispersion occurring over the transmissionline is already known are identical to the aforesaid actions to beperformed in the optical transmitter 1 in accordance with the firstembodiment shown in FIG. 2.

When the magnitude of wavelength dispersion of the transmission line isunknown, the phase shifter 66 shifts the phase of a phase shifter outputsignal by a shift that ranges from −2π/3 to 2π/3. At this time, thephase shift Δφ is determined so that a half-clock frequency signalextracted by the half-clock extracting circuit 73 included in theoptical receiver 7 will be maximized. Otherwise, nothing is performedand control is passed to the next processing.

In the optical receiver 7, the half-clock frequency signal that is acomponent of a reception signal is detected. A shift to be provided bythe phase shifter 66 and a gain to be produced by the gain variationdevice 65 are controlled in order to maximize the voltage, current, orpower level of the detected signal (steps S1 to S20 in FIG. 9).

The phase shifter 66 shifts the phase of the phase shifter output signalby Δθ (step S17 in FIG. 9). The phase is adjusted in order to maximizethe half-clock frequency signal (steps S15 to S17 in FIG. 9).

A gain to be produced by the gain variation device 65 is then changed(step S18 in FIG. 9), and adjusted in order to maximize the half-clockfrequency signal (steps S18 to S20 in FIG. 9) After the maximum level ofthe half-clock frequency signal is determined, the half-clock frequencysignal is monitored and kept controlled so that it will be retained atthe maximum level (step S15 to step S20 in FIG. 9).

Included in the present embodiment is the means for extracting a clockcomponent (half-clock frequency component) having a half of the clockfrequency of a data signal from an electric signal received by thereceiver. Also included is the means for controlling the phase of acarrier component of an optical transmission signal or optical receptionsignal so that the voltage, current, or power level of the extractedclock component will be maximized. Even when the magnitude of wavelengthdispersion occurring over a transmission line varies time-sequentially,the variation can be compensated readily. Eventually, waveformdeterioration derived from dispersion can be compensated readily.

The foregoing advantage will be explained using a formula. When thephase of part of light output from a light source is shifted by Δφ, thespectrum S(f) of an electric signal resulting from photoelectricconversion performed by a receiver is, as mentioned above, expressed asfollows:

S(f)=α×2cos(Δφ/2)

cos(πf ²λ² D/c−Δφ/2)G(f)+γ(f)

When the eye openings in an eye pattern depicting a reception signalshrink along with deterioration in a transmission characteristic, ahalf-clock frequency component diminishes. Therefore, the half-clockfrequency component is monitored, and Δφ is changed properly in order tocompensate deterioration in the half-clock frequency component.Consequently, even when the magnitude of wavelength dispersion occurringover a transmission line varies time-sequentially, the variation can becompensated readily. Eventually, deterioration in the waveform of alight signal derived from dispersion can be compensated readily.

According to the present invention, since the aforesaid method isemployed, the spectrum of a conventionally employed light signal willnot be widened. Moreover, dispersion can be compensated relative to eachchannel. The method will therefore be effectively implemented inwavelength division multiplexing communication.

Consequently, an optical transmission system resistive to wavelengthdispersion can be provided. Even when wavelength dispersion occurringover a transmission line varies time-sequentially, the variation can becompensated readily. Eventually, deterioration in the waveform of alight signal derived from dispersion can be compensated readily. In awave length division multiplexing communication system, dispersion canbe readily compensated relative to each channel. Besides, the spectrumof a transmission signal can be held unchanged.

FIG. 10 is a block diagram showing the configuration of an opticaltransmission system in accordance with the sixth embodiment of thepresent invention. Referring to FIG. 10, the optical transmission systemin accordance with the present embodiment comprises an opticaltransmitter 8 and an optical receiver 9.

The optical transmitter 8 comprises, similarly to the opticaltransmitter 1 in accordance with the first embodiment, a light source 81and an interference unit 82. The interference unit 82 consists of afirst optical power divider 83, an intensity modulator 84, a gainvariation device 85, a phase shifter 86, an optical coupler 87, and aphase controller 88.

The optical receiver 9 has a different configuration from the opticalreceiver 7 of the optical transmission system in accordance with thefifth embodiment of the present invention. The optical receiver 9comprises a first photoelectric conversion unit 91, a regeneration andidentification circuit 92, a half-clock extracting circuit 93, a maximumlevel detection circuit 94, a second optical power divider 95, and asecond photoelectric conversion unit 96.

In the optical receiver 9, a light signal input over a transmission line89 is divided into two by the second optical poser divider 95. One ofresultant light signals is converted into an electric signal by thefirst photoelectric conversion unit 91, and then converted into a clocksignal and a regenerated and identified output signal by theregeneration and identification circuit 92. The other light signal isconverted into an electric signal by the second photoelectric conversionunit 96, and then input to the half-clock extracting circuit 93.Consequently, a half-clock frequency signal is extracted.

The half-clock frequency signal extracted by the half-clock extractingcircuit 93 is input to the maximum level detection circuit 94. A controlsignal is then produced. The configuration of the interference unit 82may be identical to that of the interference unit 22 employed in thesecond embodiment, the interference unit 32 employed in the thirdembodiment, or the interference unit 42 employed in the fourthembodiment.

The phase of a carrier component of an optical transmission signal oroptical reception signal is shifted in order to minimize a phasedifference between the carrier component of a light signal transmittedover the transmission line 69 or 89 and the other frequency component.Deterioration in the waveform of the light signal derived fromwavelength dispersion and manifested during photoelectric conversion istherefore suppressed. Consequently, the resistivity of the opticaltransmission system to dispersion of a transmission signal can beimproved markedly. The results of numerical calculation graphicallyshown in FIG. 3 demonstrate that the optical transmission system isresistive to dispersion whose magnitude is twice as large as that ofdispersion to which conventional optical transmission systems areresistive.

Moreover, the optical receiver 7 or 9 includes a means for detecting ahalf-clock frequency signal component of a received electric signal andfor giving control to maximize the voltage, current, or power level ofthe half-clock frequency signal component. When wavelength dispersionoccurring over the transmission line 69 or 89 varies time-sequentially,the variation can be compensated readily. Eventually, deterioration inthe waveform of a light signal derived from dispersion can becompensated readily.

Furthermore, dispersion can be readily compensated relative to eachchannel. Even when the method in accordance with the present inventionis implemented, the spectrum of a light signal remains unchanged. Forthis reason, the method will be effectively implemented in wavelengthdivision multiplexing communication.

The above description is concerned with a light modulation method. Thepresent invention can be implemented in various modes. The opticaltransmitters 1 to 3, 6, and 8 include the intensity modulators 14, 24,34, 64, and 84. Alternatively, any of other various types of lightmodulators or modulation methods may be adopted. The present inventionwill prove effective when implemented in a configuration including, forexample, a combination of a phase modulator and an intensity modulator.

The optical transmitters 1 to 4, 6, and 8 in accordance with the presentinvention transmit data using one channel. The method in accordance withthe present invention will prove effective even when multiple channelsare used.

In the optical transmitters 1 to 3, 6, and 8 in accordance with thepresent invention, a shift to be provided by the phase shifter 16, 26,36, 66, or 86 and a gain to be produced by the gain variation device 15,65, or 85 are changed using a control signal. The present invention isnot limited to this mode. Alternatively, the shift and gain may be fixedto certain optimal values.

Furthermore, according to the light modulation method of the presentinvention, the interference unit 12, 22, 32, 62, or 82 having the phaseshifter 16, 26, 36, 66, or 86 shifts the phase of a carrier componentamong all the frequency components of alight signal. The presentinvention is not limited to this mode. As long as the phase of thecarrier component alone is shifted, the interference unit may have anycomponents. For example, two light sources having the same resonantfrequency may be employed. In this case, continuous-wave light emanatingfrom one of the light sources is modulated using an electric signal inorder to produce a light signal. The phase of continuous-wave lightemanating from the other light source is shifted. Both the light signalsare then joined.

Any circuit elements may be adopted for the circuits and devicesincluded in the first to sixth embodiments of the present invention aslong as they have the capabilities of the aforesaid circuits anddevices.

As described so far, according to the present invention, an opticaltransmitter for transmitting a modulated light signal shifts the phaseof light used as a carrier. Owing to the configuration, the precodercircuit that is essential to the conventional optical duobinary methodneed not be included. According to the present invention, an opticaltransmission method for optically transmitting an NRZ or RZ signal thatcan be improved the resistivity of an optical transmission system todispersion can be realized.

While this invention has been described in connection with certainpreferred embodiment, it is to be understood that the subject matterencompassed by way of this invention is not to be limited to thosespecific embodiments. On the contrary, it is intended for the subjectmatter of the invention to include all alternative, modification andequivalents as can be included within the spirit and scope of thefollowing claims.

What is claimed is:
 1. An optical transmitter comprising: an opticalpower divider for dividing input light into first and second continuouswave (CW) lights; a light modulator for modulating said first CW lightaccording to a data signal so as to output modulated light; a phaseshifter for shifting the phase of said second CW light so as to outputphase-shifted light, wherein said phase shifter is adjustable to vary amagnitude of phase shift of said second CW light based on a magnitude ofwavelength dispersion occurring over a transmission line; and an opticalcoupler for coupling said modulated light and said phase-shifted light.2. The optical transmitter according to claim 1, wherein a dividingratio at which said optical power divider divides light is determined sothat the mean power of said modulated light and the mean power of saidphase-shifted light will be significantly equal to each other.
 3. Theoptical transmitter according to claim 1, further comprising at leastone gain variation device inserted in at least one of a plurality ofpaths extending from said optical power divider to said optical coupler.4. The optical transmitter according to claim 1, wherein said lightmodulator comprises an intensity modulator.
 5. The optical transmitteraccording to claim 1, wherein said light modulator comprises a phasemodulator.
 6. The optical transmitter according to claim 1, wherein saidoptical power divider, light modulator, phase shifter, and opticalcoupler are integrated into one optical substrate.
 7. The opticaltransmitter according to claim 1, further comprising a light source forgenerating said input light.
 8. An optical transmission system,comprising: an optical transmitter for outputting a light signal; anoptical transmission line over which said light signal is transmitted;and an optical receiver for receiving said light signal output over saidtransmission line, wherein said optical transmitter comprises an opticaltransmitter set forth in claim 1; said optical receiver; comprising afirst photo-detector for converting a first light signal to an electricsignal; a clock extracting circuit for extracting a predeterminedfrequency component of the electric signal and outputting it as a clocksignal; and a maximum level detection circuit for detecting the maximumlevel of said clock signal; said optical transmission line includes acontrol signal transmission line over which said maximum level istransmitted to said optical transmitter; and said optical transmitterfurther includes a phase controller for outputting a control signal withwhich a phase shift to be provided by said phase shifter is adjusted inorder to maximize said maximum level.
 9. The optical transmitteraccording to claim 1, further comprising a phase controller forcontrolling said phase shifter.
 10. An optical transmission system,comprising: an optical transmitter for outputting a light signal; anoptical transmission line over which the light signal is transmitted;and an optical receiver for receiving the light signal output over saidtransmission line, wherein said optical transmitter comprising anoptical power divider for dividing input light into first and secondcontinuous wave (CW) lights; a light modulator for modulating said firstCW light according to a data signal so as to output modulated light; aphase shifter for shifting the phase of said second CW light so as tooutput phase-shifted light, wherein said phase shifter is adjustable tovary a magnitude of phase shift of said second CW light based on amagnitude of wavelength dispersion occurring over a transmission line;and an optical coupler for coupling said modulated light and saidphase-shifted light; said optical receiver comprising a firstphoto-detector for converting a first light signal to electric signal, aclock extracting circuit for extracting a predetermined frequencycomponent of the electric signal and outputting it as a clock signal anda maximum level detection circuit for detecting the maximum level ofsaid clock signal, and an optical power divider for dividing a secondlight signal to be input so as to output said first light signal andsecond division light, and second photo-detector for converting thelight into an electric signal; said optical transmission line includes acontrol signal transmission line over which said maximum level istransmitted to said optical transmitter; and said optical transmitterfurther includes a phase controller for outputting a control signal withwhich a phase shift to be provided by said phase shifter is adjusted inorder to maximize said maximum level.
 11. An optical transmissionmethod, comprising the steps of: dividing light output from a lightsource into at least two division lights; adjusting a magnitude of phasebased on a magnitude of wavelength dispersion occurring over atransmission line; shifting the phase of one of said division lights soas to produce phase-shifted light; modulating the other division lightso as to produce modulated light; and coupling said phase-shifted lightand modulated light.
 12. The optical transmission method according toclaim 11, wherein said producing phase-shifted light includes a step ofadjusting the power of at least one of said division lights andphase-shifted light.
 13. The optical transmission method according toclaim 11, wherein said producing modulated light includes a step ofadjusting the power of at least one of said division lights andmodulated light.